Depleting cheap fossil fuel reserves and a pressing need for greenhouse gas (GHG) emission reduction are two major technico-economic challenges. Thus, it is quite urgent to develop cost effective, clean and renewable sources for both energy and chemical needs. Microalgae is actively investigated as a long term solution to cover these needs. Microalgae has a potential of producing up to 4,000 Gallons of oil per acre per year. This production rate is more than an order of magnitude higher than any other biofuel source. However currently used production and harvesting processes of microalgae are energy intensive and relatively costly. Except for a few high value nutrients, proteins and other byproducts, microalgae based biofuels are not commercially viable. Furthermore, current energy intensive harvesting processes give rise to significant CO2 emissions.
There are five important processing steps required to obtain biofuels and/or chemicals from microalgae. Step #1 involves cultivating microalgae to produce more microalgae. Following the cultivation step, microalgae is collected and dewatered in Step #2 leading to concentrated dilute microalgal suspensions having TSS (total solid suspensions) content in a range from 0.5 to 5%. More extensive dewatering process combining one or more techniques that include centrifugation, flocculation, filtration and screening, gravity sedimentation, flotation and electrophoresis increases the TSS content up to around 10-20% TSS (Step #3). In Step #4, a drying process gives rise to a TSS of at least 25%. In the last step (Step #5), extraction processes are undertaken to produce the final product. In some cases, product extraction may be undertaken before the drying step. For example, in the case of anaerobic digestion primary dewatering is sufficient.
Following the cultivation step using a photobioreactor, the yield is often in the range of a maximum of about 1 kg dry weight/day/m3. The average TSS content is about 0.05% (Step #1). The large amount of water comprises extracellular and intracellular water. Depending on the requirements for drying (Step #4) and extraction (Step #5) and the targeted list of final products and byproducts, intracellular water removal may take place at different stages.
Different processing technologies are used for transforming the slurry to a sludge/cake and then to a dry state. Depending on the dewatering process, these industrial processes may also give rise to low specific production yield. The yield of these dewatering processes should be high while using minimum amount of energy.
A drying process that allows the completion of the harvesting process could increase the TSS content to about 75%. Dehydration faces two challenges related to algae degradation and loss of valuable chemicals and high energy cost. In the case of extraction, the following processes are often used: mechanical crushing (expeller press), solvent (hexane, benzene) extraction, supercritical CO2, enzymatic hydrolysis, microwave, cavitation and cellular decompression.
Three different algae cultivation methods are used including raceway pond, tubular photobioreactor and flat plate photobioreactor. Raceway pond has the lowest capital cost with the lowest energy input. However raceway pond uses significant land area and water with poor biomass productivity. Furthermore, raceway pond systems are limited to a few strains of algae with less control over the cultivation conditions. A photobioreactor can be generally described as an enclosed, illuminated culture vessel designed for controlled biomass production of phototrophic liquid cell suspension cultures. Tubular photobioreactors have been developed to increase biomass productivity by providing a large specific illumination surface area and more control of cultivation conditions. Capital cost of tubular reactors is relatively higher than raceway pond. They also present several challenges related to fouling, presence of oxygen and CO2, and gradients of pH values. They require large land area, although less than the raceway pond. In the third method, flat plate photobioreactors provide the highest biomass productivity, although illumination conditions are less than optimal. Flat plate photobioreactors are cheaper to produce, but they are difficult to scale-up with significant temperature control challenges. Per unit mass of produced algae, flat plate photobioreactors are cost effective.
Cultivation and dewatering represent two significant challenges for implementing commercial processes. These steps are critical for implementing an algae-based manufacturing of chemicals (nutrients, proteins) and biofuel (biodiesel) products. Unless the financial and energy costs of these two steps are significantly reduced, the commercial viability of biodiesel-based microalgae is questionable. However, high value byproducts such as nutrients and protein obtained from microalage are currently commercially viable.
The majority of dewatering techniques are based on water removal from the algae suspension. Electrochemical processes including electrodeposition (ED), electrocoagulation (EC), electroflotation (EF) and electrooxidation (EO) could be used for algae removal. Reducing the energy cost in the algae dewatering and drying processes while maintaining high yield output are commercially important. For example, electroflotation presents several attributes for large scale algae removal. Indeed, large scale algae removal from waste using electroflotation has been demonstrated. They do not require additional chemical flocculants or a sacrificial electrode and give rise to high yield (90% or more). Adding chemicals makes the downstream processes even more complicated and expensive. Electrodeposition and electroflotation face other specific challenges related mostly to additional capital cost.
Combining two or more of the processing steps discussed above into a single step would not only reduce capital cost but would also reduce cost of operation and maintenance (O&M). In particular, combining cultivation and dewatering using a single apparatus and/or process could allow high production yield with reduced production cost.
There remains a need for a photobioreactor design and process that meets one or more of the aforementioned challenges.